Control circuits for space discharge devices



jam. 3, 1933. F. B. LLEWELLYN CONTROL CIRCUITS FOR SPACE DISCHARGEDEVICES Filed April 12. 1930 1 v, L mm m "I M A n i 3 Y% B F. n m m w EF. M m Q T0 Patented Jan. 3, 1933 UNITED STATES PATENT OFFEQE FREDERICKB. LLEWELLYN, OF MON'ICLAIR, NEW JERSEY, ASSIGNOR TO BELL TELE- PHONELABORATORIES, INCORPORATED,

NEW YORK OF NEW YORK, N. Y., A CORPORATION OF CONTROL CIRCUITS FOR SPACEDISCHARGE DEVICES Application filed April 12,

This invention relates to circuit arrangements for the eflicient andquiet operation of space discharge devices and particularly to theoperation of these devices as modulators and detectors.

In the operation of a space discharge device it is generally desirableto secure a high degree of amplification and a large output of usefulpower together with relatively little generation of disturbing currentswhich will give rise to noises in a receiver connected to the device.

It is well known that the disturbing effect of noise producing currentsis dependent upon their relative intensity compared with the intensityof the useful currents with which they are commingled in a receivingdevice. It is evident that any measures taken to re duce noises whichoriginate in space discharge devices and their attached circuits must,in order to be effective, either increase the intensity of the usefulcurrents or not reduce them to the same extent that the noise producingcurrents are reduced.

In the present invention the desired eflicient and quiet operation of aspace discharge device is secured by a particular adjustment of thedirect current biasing potentials which areapplied to the variouselectrodes of the device together with a readjustment of the intensitiesof the alternating currents applied to the device. The manner in whichthese adjustments provide for increased eificiency and quiet operationwill be pointed out more particularly hereinafter.

In the proceedings of the Institute of Radio Engineers for February1930, Volume 18, N0. 2, pages 243 to 265, I have published the resultsof a study of the causes of noise in vacuum tubes and attached circuits.The principal sources of noise which I have found and those which aremost susceptible of control are the small shot effect in vacuum tubesand the thermal agitation of conductors.

The small shot effect is due to the existence of slight irregularitiesin the stream of electrons passing from the cathode to the anode ofavacuum tube, giving rise to fluctuations in the plate current. Thefluctuations are due to the arrival of electrons at the plate at a 1930.Serial No. 443,696.

non-uniform rate. This effect is most pronounced when a vacuum tube isoperated in a condition in which temperature saturation is not complete.

The thermal agitation of electricity in a conductor is a state due tothe irregular motion of the electric charges in the conductor and isproduced by the heat energy in the conductor. This irregular motion ofthe charges produces slight irregularities in the current through theconductor and gives rise to noises in the receiver. The effect ofthermal agitation is greater in large resistances than in small ones andincreases with the temperature at which the resistance is operated.

In many commercial tubes the amount of noise is reduced to asatisfactory amount by operating the tube in a condition of temperaturesaturation to reduce the small shot effect and by using fairly largedirect current plate potential to reduce the plate filament resistanceand its accompanying thermal agitation to a small value.

In certain tube structures it has been found that the condition oftemperature saturation is not readily obtained with the high values ofdirect current plate potential commonly used. The four-electrode type orscreen grid tube which is at present available commercially isparticularly disadvantageous in this respect. Tubes of this type aregenerally sufliciently quiet in operation when employed as amplifiers,for then the useful currents are amplified to a sufficient intensity sothat the noise producing currents are relatively weak and henceunobjectionable. lVhen the tube is used as a modulator or detector,however, the degree of amplification obtainable is less and the noiseproducing currents are relatively strong, requiring the use of specialmeasures to reduce their relative intensity.

In order to improve the temperature saturation in a tube of the typeabove mentioned and thereby decrease the small shot effect I reduce thevalue of the direct current potentials employed. I use a relativelysmall positive bias on the screen grid electrode and preferably apply nodirect current potential whatever to the plate. In reducing the directcurrentbiasing potentials the degree of am- V plification obtained in atube is decreased.

t is possible, however, to compensate to a large degree for thisdecrease of amplification by correspondingly increasing the. in-

tector this loss of amplification is notof great importance. While thereduction of amplification necessarily means that the intensity of theuseful currents is reduced, it has been found that the intensity of thenoise produc I ing currents isreduced to a still greater extent. Theresult is a net improvement in the ratio of useful currents tonoiseproducing currents. p

' I The invention will now be described indetail with reference to thedrawing in which Fig. 1 shows an embodiment of the invention employing afour-electrode space discharge device. Fig. 2 shows the application ofthe invention to a three-element tube and Fig. 3 shows its applicationto a two-element tube while Fig. 4 shows theoretical" curves which willbe employed in explaining the invention.

Referring to Fig. 1, the terminals 1 and 2 arerinput terminals to whichmay be con nected a source of incoming electrical waves or currents.Theinput circuit 1, 2 is coupled by means of a transformer Sto an inputtuned circuit 4. *The tuned circuit 4 is connected to the input side ofa fourele'ctrode space dischar e device 5 havin a cathode 6 a an anodeor plate 8, and a control grid 7,

One side of'the circuit 4 is screen grid 9.

- connected'to the cathode Giandthe other side isconnected to thecontrol grid 7. The screen grid electrode 9 is connected to the positiveterminal of a battery 10. An adjustable connector '15 is connected toanother portion of the battery 10 and to the cathode 6. By ad justmentof the connector a positive direct current potential of any desiredvalue may be impressed upon the screen grid 9.

' A'local source of alternating currents for us'e in heterodynereception of the incoming waves is provided at 16. This source isadjustable as to frequency by any suitable means as indicated by thearrow in the diagrammatic representation. The source 16 isconnected tothe plate circuit of the tube 5-through an adjustable potentiometer 50and any suitable coupling means, which latter is represented by thetransformer 17. An output circuit 18 is provided which is tuned for thefrequency of the useful current to be derived from the tube 5. Thecircuit 18 is coupled by means of a transformer 19 to theoutputterminals and 26. a

The plate 8 is preferably operated without any direct current potentialbeing applied thereto, but a small potential may be applied withoutdeparting from the scope of the invention. By varying the position ofthe con tact 15 in the screen grid circuit and the intensityofthe'alternatin'g current from the which gives a large degree ofamplification and a large output of useful current together with arelatively small amount of disturbing currents or noises. i I r I Thecircuit shown in Fig. 2 is similar to that shownin Fig. l 'ericept thata three electrode tube 20 is employedin place of the four-electrode tube51 The plate 8110f. the tube 20 is provided withan adjustable biasingbatterylt) and {is preferably operated with relatively little or nodirect current biasing potential. V The effective potential of battery10 and the intensityof the alternating current from the local'sourc'e 16maybe adjusted to such values that the circuit will give a maximumamplification and a large amount of useful current with a minimum amountof noise.

InFig. 3a two-elementtube fis-illustrated. The input circuit ,4 isconnected in series with the output'circuit 18. As in the caseofthecircuit shown in Fig. 2 theeifec 'tive potentialof battery 40and theintensity 01"- the current from the local source 16 may be adjusted toobtain the most quiet and efiicient operation of the system.;

The principle underlying the adj ust-ment bf i ent values of the directcurrent biasing potential of the tube in decreasing order.

Each of these curves has a rising portion for the small values of thecathode temperature and a nearly horizontal portion, for the highercathode temperatures. The r sing portion of the curve represents acondition in which the plate electrode is drawing electrons directlyfrom the cathode and is a condition in which temperature saturation doesnot exist. In this condition any irregularity in the emission ofelectrons from the cathode produces a corresponding irregularity in theplate current. In other words, this region'is one in which the smallshoteffect is most pronounced and gives rise to noise in the plate circuit.

The horizont'al portion of the curve represents a condition oftemperature saturation. In this condition a change in the cathodetemperature has practically no effect upon the intensity'of the platecurrent. At these high temperatures the cathode emits electrons greatlyin excess of'the number'required to constitute the plate current. Theexcess elec-' these circumstances is composed of electrons drawn fromthe space charge and not directly from the cathode. The irregularitiesin the stream of electrons emitted from the cathode are not transmitteddirectly to the plate and consequently it appears that the plate currentis made more uniform by operating the tube under the condition oftemperature saturation.

The curves 21, 22, 23 and 24 represent the manner in which the tubenoises depend upon the cathode temperature. Curve 2 1 corre sponds tothe same direct current plate potential as the curve 11. In the same Waycurves 22 and 12 correspond and also 23 corresponds with 13 and 24 with14. Each of the curves 21, 22, 23 and 24 shows a maximum amount of noisein the region where the plate current curve is rising and each of thesecurves also falls to a relatively small value at the higher cathodetemperatures.

The maximum value of noise is in each case due to the small shot effectwhile the noise observed at the higher cathode temperatures,

that is, in the region of temperature saturation depends principallyupon the thermal agitation within the tube. Since the noise of thermalagitation increases when the plate filament resistance increases, thenoise curve 21 which corresponds to the smallest value of plate filamentresistance falls to a lower value than the curves 22, 23 and 24. Thelatter curves, in order, come down to increasingly higher final valuesbecause they correspond to larger values of plate filament resistance.

A tube which cannot be operated in a condition of temperature saturationwill have its normal operating temperature represented approximately bythe value T The vertical line corresponding to the temperature Tintersects the noise curves 21, 22, 23 and 24 at the points 31, 32, 33and 34 respectively. It will be seen that as the direct current biasingpotential is reduced the values of noise obtained in the tube willcorrespond to the ordinates of the points 31, 32, 33 and 34. A minimumamount of noise is obtained in the region of the points 33 and 34. Byadjusting the effective potential of the battery 10 in Fig. 1 or battery40 in Figs. 2 and 3 an operating condition corresponding to the point 33may readily be obtained.

As hereinbefore mentioned, the decreased values of direct currentbiasing potential corresponding to the operating conditions of points 33and 34 are accompanied by a reduction of amplification and a lower intensity of useful current. The noise, however, is reduced in a greaterdegree than is the useful current so that there is a materialimprovement, that is, a diminution in the net disturbing effect of thenoise.

What is claimed is:

1. A circuit comprising a space discharge device having an anode,a'cathode, a control grid and a screen grid, the anode being at the samedirect current potential as the cathode, means for applying a positivedirect current bias to said screen grid, a source of alternating currentcoupled to said anode, and means for adjusting the intensity of saidalternating current whereby the ratio of useful currents to noiseproducing currents in said device is increased. I

2. A circuit comprising a space discharge device having an anode, acathode, a control grid and a screen grid, the anode being at the samedirect current potential as the cathode, means for applying a positivedirect current potential to said screen grid, a source of alternatingcurrent coupled to said anode, and means for adjusting the intensity ofalternating current from said source.

3. A circuit comprising a space discharge device having an anode, acathode, a control grid and a screen grid, the anode being at the samedirect current potential as the cathode, means for applying a positivedirect current bias to said screen grid, a source of alternatin gcurrent coupled to said anode, input and output circuits for said spacedischarge device, and means for adjusting the bias of,said screen gridwhereby the ratio of useful currents to noise producing currents in saidoutput circuit is increased.

4. A circuit comprising a space discharge device having an anode, acathode, a control grid and a screen grid, the anode being at the samedirect current potential as the oathode, means for applying a positivedirect current bias to said screen grid, a source of alternating currentcoupled to said anode, input and output circuits for said space discharge device, means for adjusting the bias of said screen grid, andmeans for adjusting the intensity of the alternating current from saidsource, whereby the ratio of useful currents to noise producing currentsin said output circuit is increased.

5. A mod ulating system comprising a space discharge device having ananode, a cathode, a control grid and a screen grid, a source of carriercurrent to be modulated, means for coupling said source to said anode, asource of modulating current connected to said control grid, an outputcircuit for utilizing modulated currents, the anode of said dischargedevice being at substantially the same direct current potential as saidcathode, and the screen grid having such a value of direct currentpotential with respect to said cathode that the ratio of useful currentsto noise producing currents in said output circuit is substantially amaximum.

6. A modulating system comprising a space discharge device having ananode, a cathode, a control grid and a screen grid. a source of carriercurrent to be modulated, means for coupling said source to said anode, asource of modulating current connected to, said control grid, anoutputcircuit for utilizing modulated cur-rents fromsaid device, said anodebeing at substantially the samedirect current potential as said cathode,and the direct current potential of said screen grid and the alternatingcurrent potential of said anode being at suchvalues respectively thatthe 7 ratio of usefulcurrents to noise producing currents in said outputcircuit is substantially a maxlmum.

IrIn wltness whereof, I hereunto subscribe my name this 11th day ofApril, 1930. r I FREDERICK B.,LLEWELLYN.

